1. Field of the Invention
This invention relates to an image formation method using electrostatic printing and an apparatus therefor, and more particularly to an image formation capable of controlling the image density and an apparatus therefor.
2. Description of the Prior Art
Recently, in the field of printing or recording, remarkable advancements have been seen in the development of higher-speed, simple and accurate recording or printing methods and further in the development of the materials used therefor. Among them, the thermosensitive photosensitive medium composed chiefly of organic silver salt eliminates the necessity of being subjected to the wet development and fixing process after exposure to an image light as has been the common photographic material sing inorganic silver salt, and this is one of the materials which have especially received attention. That is, the thermosensitive photosensitive medium composed chiefly of organic silver salt permits the adoption of the so-called dry development which can accomplish development simply by heating that medium after it has been exposed to image light in a conventional manner. Therefore, as compared with the cases where wet development is required, this medium is advantageous in that the processing liquid for development and fixation is not required and development can be accomplished simply by applying heat to the material after the image exposure has been applied thereto and moreover, there is no necessity of fixation in particular.
The thermosensitive photosensitive medium may usually comprise a back-up member such as a film or sheet of plastic or paper or like material and an image formation layer chiefly of organic silver salt provided on said back-up member as by coating or like means. More particularly, such photosensitive medium may be obtained by dispersing organic silver salt, reducing agent and a small amount of halide with respect to the organic silver salt through a binder such as electrically insulative material and providing these materials on the back-up member. The thermosensitive photosensitive medium so composed may be subjected to image exposure, whereafter (or simultaneously therewith) it may be heated and developed to form a silver image thereon. The electrostatic printing is to electrostatically produce printed matter by using the photosensitive medium prepared in the manner described above. Typical organic silver salts unable for such photosensitive medium include organic acid silver salts such as silver behenate, silver arachidate, silver stearate, silver palmitate, silver myristate, silver laurate, silver caprylate, silver uranate, silver hydroxystearate, silver acetate, etc.; and organic silver compounds such as silver benzoate, silver phthalazinone, silver benztriazole, silver saccharin, silver 4-n-octadecyloxydiphenyl-4-carboxylate, silver-0-aminobenzoate, silver-acetoamidobenzoate, silver furoate, silver camphorate, silver p-phenylbenzoate, silver phenyl acetate, silver salicylate, silver butyrate, silver terephthalate, silver phthalate, silver acid pathalte, etc.
The reducing agent includes hydroquinone, methylhydroquinone, chlorohydroquinone, bromohydroquinone, catechol, pyrogallol, methylhydroxynaphthalens, aminophenol, 2,2'-methylene bis(6-t-butyl-4-methylphenol), 4,4'-butylidone bis(6-t-butyl-3-methylphenol), 4,4'-bis(6-t-butyl-3-methylphenol), 4,4'-thio bis(6-t-butyl-methylphenol), 2,6-di-t-butyl-p-cresol, 2,2'-methylene bis(4-ethyl-6-t-butylphenol), phenidone, metol, 2,2'-dihydroxy-1,1'-binaphthyl, 6,6'-dibromo-2,2'-dihydroxy-1,1'-binaphthyl, 6,6'-dinitro-2,2'-dihydroxy-1,1'-binaphthyl, or bis(2-hydroxy-1-naphthyl) methane, or a mixture thereof. The halides for imparting photosensitivity to the organic silver salt include various inorganic halides (X.dbd.CI.Br.I) such as NH.sub.4 X, CrX.sub.2, IrX.sub.4, InX.sub.4, CoX.sub.2, CdX.sub.2, KX, HX, SnX.sub.2, SnX.sub.4, SrX.sub.2, SO.sub.2 X.sub.2, TiX.sub.3, TiX.sub.4, CuX.sub.2, NaX, PbX.sub.2, NiX.sub.2, PdX.sub.2, MgX.sub.2, Al.sub.2 X.sub.2, MnX.sub.2, BaX.sub.2, KAuX.sub.4, HAuCl.sub.4, BiX.sub.3, CsX, FeX.sub.3, AgX, etc.
The action and mechanism of the above-mentioned halides contributing to the image formation can not fully accurately be explained as yet, whereas among these halides, silver halide is known as a substance which produces free silver upon exposure to light, said free silver forming a developing core during development and expediting the liberation of silver from organic silver salt to form a silver image. As regards the other halides than silver halide, these react to organic silver salt during the formation of the thermosensitive photosensitive medium to create silver halide and when exposed to light, the silver halide produces free silver as already described, and such free silver forms a developing core to form a silver image on the exposed portion.
The insulative medium may suitably be any of various resins such as polystyrene resin, polyvinyl chloride resin, phenolic resin, polyvinyl acetate resin, polyvinyl acetal resin, epoxy resin, xylene resis, alkyd resin, polycarbonate resin, polymethyl methacrylate resin, polyvinyl butyral resin, gelatin resin, polyester, polyurethane, synthetic rubber, polybutene, polyvinyl acetate, etc.
A plasticizer may be added, as required. The plasticizer may be dioctyl phthalate, tricresyl phosphate, diphenyl chloride, methyl naphthalene, p-terphenyl, diphenyl or the like.
The electrostatic printing process using such photosensitive material comprises first charging the photosensitive medium to form thereon an electrostatic latent image corresponding to the silver image, developing such electrostatic latent image into a toner image, thereafter transferring the toner image to a transfer medium and fixing the toner image.
The quantity of charge imparted during the step of charging is determined by the charge-retaining power of the silver image portion produced by subjecting the thermosensitive photosensitive medium to the exposure and heating steps, and the charge retaining power may be taken as the electrical resistance, the electrostatic capacity or the photographic density of the silver image portion. More specifically, if the electrical density is small, the charge retaining power may be said to be great. As already described, if the charge retaining power is always constant under a predetermined exposure and a predetermined heating condition, each step in the electrostatic printing may be fixed to a predetermined condition.
However, one of the characteristics of the thermosensitive photosensitive medium is that it can not always have a constant charge retaining power in spite of being subjected to a predetermined exposure, under the influence of the environment in which it is stored, the influence of the temperature and humidity at which it is used, and the influence of the instability of the heat source during the heating step. Therefore, if the charge retaining power is greater than a value expected, a greater quantity of charge will be retained by such photosensitive medium due to charging, so that a greater quantity of toner will be deposited on the medium during the step of toner development. Should the portions of the photosensitive medium on which toner should not be deposited have a greater charge retaining power from such a cause, these portions will be developed during the step of toner development, thus resulting in deposition of toner on these portions, namely, creation of fog which will mean stained printed matters.
Conversely, a smaller charge retaining power would undesirably fail to provide the toner image with a contrast.